Design Guidance for J-Turns on Rural High-Speed Expressways

Abstract

Owing to the J-turn’s safety effectiveness, it has become a viable alternative to replace high-crash, two-way, stop-controlled intersections on high-speed expressways. National guidance on the design of J-turns on high-speed highways is limited. What is the safety effect of spacing between the intersection and the U-turn? Under what circumstances is the provision of acceleration lanes recommended? This study answered these questions through a safety assessment on the basis of ( a) an examination of crashes that occurred at 12 J-turn sites in Missouri and ( b) a simulation-based assessment of the effect of various design variables and traffic flows. The crash review revealed the proportions of the five most frequently occurring crash types at J-turn sites: ( a) major road sideswipe (31.6%), ( b) major road rear-end (28.1%), ( c) minor road rear-end (15.8%), ( d) loss of control (14%), and ( e) merging from U-turn (10.5%). The crash rates, accounting for exposure, decreased with the increase in the spacing to the U-turn for sideswipe and rear-end crashes; J-turns with a spacing of 1,500 ft or greater experienced the lowest crash rates. The crash rates were lower for J-turn sites with acceleration lanes for minor road traffic merging onto the major road than for sites without acceleration lanes. A calibrated simulation model analysis revealed that the presence of acceleration lanes reduced conflicts for all volumes and designs, including low volumes. The simulation analysis also reinforced the crash analysis results that safety improved with an increase in spacing between the main intersection and the U-turn.